Thermoelectric transport of semiconductor full-Heusler VFe2Al

Abstract

The full-Heusler VFe₂Al has emerged as an important thermoelectric material in its thin film and bulk phases. VFe₂Al is attractive for use as a thermoelectric materials because of it contains only low-cost, non-toxic and earth abundant elements. While VFe₂Al has often been described as a semimetal, here we show the electronic and thermal properties of VFe₂Al can be explained by considering VFe₂Al as a valence precise semiconductor like many other thermoelectric materials but with a very small band gap (E_g = 0.03 ± 0.01 eV). Using a two-band model for electrical transport and point-defect scattering model for thermal transport we analyze the thermoelectric properties of bulk full-Heusler VFe₂Al. We demonstrate that a semiconductor transport model can explain the compilation of data from a variety of n and p-type VFe₂Al compositions assuming a small band-gap between 0.02 eV and 0.04 eV. In this small E_g semiconductor understanding, the model suggests that nominally undoped VFe₂Al samples appear metallic because of intrinsic defects of the order of ∼10²⁰ defects per cm⁻³. We rationalize the observed trends in weighted mobilities (μ_w) with dopant atoms from a molecular orbital understanding of the electronic structure. We use a phonon-point-defect scattering model to understand the dopant-concentration (and, therefore, the carrier-concentration) dependence of thermal conductivity. The electrical and thermal models developed allow us to predict the zT versus carrier concentration curve for this material, which maps well to reported experimental investigations.